Our proposed research is to explore neural mechanisms of complex-sound processing for specialized auditory function (biosonar) and also for common auditory function (communication). Our ultimate goal is the thorough understanding of the neural mechanisms for the processing of complex acoustic signals important for a species. The mustached bat emits short complex sounds for biosonar and long complex calls for communication. We have demonstrated that certain types of biosonar information are extracted by neurons sensitive to different combinations of signal elements and are systematically represented in separate areas of the auditory cortex. Certain response properties of these combination-sensitive neurons in the cortex are created in subthalamic nuclei, but combination sensitivity itself is created in the medial geniculate body. The processing of biosonar information is parallel-hierarchical. These findings are very important for understanding the neural mechanisms for processing complex sounds in general. Thus, one of our major aims is to extend our findings by exploring synaptic mechanisms for creating the response properties of combination-sensitive neurons, signal processing within and across individual cortical areas, and the upper limit in specialization (complexity of response properties) of single neurons. Mustached bats frequently emit communication calls which are quite different from biosonar signals, but are similar to calls of other mammals in spectral and temporal structures. Since one of the two major functions of the auditory system is to process the communication calls, we want to explore how the calls are processed in the auditory cortex, and whether the call-processing system overlaps with the biosonar-signal-processing system. To explore these important problems, we will examine response properties of single neurons in the central auditory system and inject small amounts of either pharmacological agents or anatomical tracers into the recordings sites to identify synaptic mechanisms or anatomical connections. The left cerebral hemisphere in humans contains the areas which are specialized for processing speech. A thorough understanding of the neural mechanisms for speech processing will not be obtained without direct physiological studies on these areas. However, the insight to it will be obtained from research on animals specialized for processing complex acoustic signals, as recently demonstrated. The acoustic signals used by the mustached bat are high in frequency, but share basic acoustic patterns with those used by many other species of mammals; including humans. Thus our proposed research will significantly contribute to an understanding of the basic neural mechanisms for processing speech sounds.